A silver halide emulsion employed in a photographic material is generally prepared by mixing a solution containing silver ion and a solution containing halogen ion(s) in the presence of a hydrophilic colloid (a precipitation step), and then, by ripening the emulsion physically, followed by, in sequence, removal of unnecessary salts from the emulsion by washing, redispersion of the emulsion, and chemical ripening of the emulsion.
The size of the silver halide grains, which is one of factors capable of controlling the sensitivity of a silver halide emulsion, is determined chiefly in the precipitation step and the physical ripening step (these steps are collectively called the grain formation step hereinafter) included in the above-described preparation process. Accordingly, addition of a silver halide solvent in the grain formation step has been carried out with the intention of increasing the grain size. The most typical silver halide solvent used is ammonia, and the grain formation method using ammonia is called the ammonia method. However, ammonia has a pungent odor, and thereby causes health and environmental control problems. In addition, when ammonia is used as a silver halide solvent, large size silver halide grains can be obtained only at a high pH value, and under high pH conditions the fog level of silver halide grains formed becomes high. Furthermore, uniformity in the sizes of silver halide grains formed is insufficient. For the above-described reasons and others, the use of ammonia as a silver halide solvent is undesirable.